This invention relates to a novel process for producing microencapsulated materials by a water-in-oil microencapsulation process, whereby microcapsules are formed which contain aqueous materials.
Numerous processes are known in the art for producing microencapsulated materials. Nearly all the known processes produce microcapsules of materials contained in a water-immiscible or insoluble material and are produced by what is termed oil-in-water microencapsulation processes. These in general involve the production of a dispersion of "oil" or organic, substantially water-immiscible liquid droplets (discontinuous phase) in an aqueous medium (continuous phase). The oil droplets contain one or more monomers or prepolymers and microcapsules are formed by subjecting the emulsion to conditions such as temperature and/or pH and/or agitation to cause polymerization of the monomers or prepolymers present in the oil phase to produce microcapsules having a polymeric shell enclosing the water-immiscible droplet phase. Such processes are described, for example, in U.S. Pat. Nos. 4,285,720 and 4,956,129. The former involves production of microcapsules of a polyurea material and the latter of an etherified urea-formaldehyde polymer.
On the other hand, there is comparatively little information available on the production of microcapsules containing an aqueous material by a water-in-oil microencapsulation process. One process which somewhat approaches a water-in-oil microencapsulation process is described in U.S. Pat. No. 4,157,983. In that process, a mixture is formed which contains an emulsifier, a water-immiscible liquid, a urea-formaldehyde prepolymer, a water-dispersible material to be encapsulated, and water. The mixture is agitated to produce a water-in-oil emulsion. The emulsion is then cured or treated to produce microcapsules by solidification of the urea-formaldehyde prepolymer resin to form a matrix encapsulating the droplets and permitting the separation of solid polymeric capsules containing the water-dispersible material. The curing or polymerization is instituted by use of an amphiphatic catalyst, that is, a catalyst which is soluble in both the water and oil phases of the emulsion. However, the products of this process are not true microcapsules but rather comprise a matrix of the urea/formaldehyde polymer containing the water dispersible material.
U.S. Pat. No. 4,534,783 discloses a process for encapsulating aqueous materials using two monomers or prepolymers.
It is an object of the present invention to provide a simple method for producing true microcapsules containing an aqueous liquid core, of relatively uniform and controlled size, which are suitable for use without further treatment.